Satellite broadcasting systems for transmitting programming content have become increasingly popular in many parts of the world. Direct Broadcasting Satellite (DBS) systems transmit television programming content, for example, to a geo-stationary satellite, which broadcasts the content back to the customers. In such a wireless broadcast environment, the transmitted programming can be received by anyone with an appropriate receiver, such as an antenna or a satellite dish.
In addition, a number of satellite broadcasting systems have been proposed or suggested for broadcasting audio programming content from geo-stationary satellites to customers in a large coverage area, such as the continental United States. Proposed systems for providing digital audio broadcasting (DAB), for example, are expected to provide near CD-quality audio, data services and more robust coverage than existing analog FM transmissions. Satellite broadcasting systems for television and radio content provide potentially national coverage areas, and thus improve over conventional terrestrial television stations and AM/FM radio stations that provide only regional coverage.
Satellite broadcasting systems transmit digital music and other information from an uplink station to one or more mobile receivers. Satellite broadcasting systems typically include a plurality of satellites and terrestrial repeaters operating in a broadcast mode. The satellites are typically geo-stationary, and are located over a desired geographical coverage area. The terrestrial repeaters typically operate in dense urban areas, where the direct line of sight (LOS) between the satellites and the mobile receiver can be blocked due to the angle of elevation and shadowing by tall buildings.
Orthogonal frequency division multiplexing (OFDM) techniques have also been proposed for use in such satellite broadcasting systems and other wireless networks. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel. It has been found that OFDM communication systems do not require complex equalizers, even at high data rates and under multipath propagation conditions. Among other benefits, OFDM communication systems provide a guard interval that absorbs the multipath distortion into the guard interval duration. As long as the arrival times of the multipath signals differ from one another by less than the guard interval, an equalizer is not necessary.
An OFDM receiver must perform timing acquisition and tracking to process data properly. FIG. 1 illustrates portions of a conventional OFDM receiver 100 directed to timing recovery. The OFDM receiver 100 implements a known Guard Interval Based (GIB) algorithm 110 that recovers timing information from the received signal. For a more detailed discussion of the GIB timing recovery algorithm 110, see, for example, Jan-Jaap van de Beek et al., ML Estimation of Time and Frequency Offset in OFDM Systems, IEEE Transactions on Signal Processing, Vol. 45, No 7, 1800-05 (July 1997) or Jan-Jaap van de Beek et al., “A Time and Frequency Synchronization Scheme for Multiuser OFDM,” IEEE J. on Selected Areas in Communications, Vol. 17, No. 11, 1900-14, (November 1999), each incorporated by reference herein.
Generally, the GIB timing recovery algorithm 110 employed by the OFDM receiver 100 identifies peaks in the maximum likelihood (ML) metric 200, shown in FIG. 2. Each peak, such as the peaks 210-216, in the ML metric 200 corresponds to the start of each OFDM frame. The peaks are present because the received samples are heavily correlated at a lag corresponding to the useful symbol duration. The timing information is extracted by a maximum index locator 120 that locates the index of the maximum correlation value in a buffer having a size corresponding to the number of samples in the OFDM frame.
While the GIB algorithm performs effectively for many applications, it suffers from a number of limitations, which if overcome, could greatly expand the reliability and accuracy of OFDM receivers. For example, since each peak 210-216 in the ML metric 200 occurs at the frame boundary and, in a dispersive channel, such as under multipath conditions, the peaks will not be ideal impulses, a given peak may start in one frame, extend over the frame boundary and end in the next frame. Thus, a maximum correlation value associated with the peak may be assigned an index at the end of the prior frame or the beginning of the next frame, causing ambiguities in the identification of frame boundaries.
A need therefore exists for improved techniques for performing timing acquisition and tracking in an OFDM receiver. A further need exists for a method and apparatus for performing timing acquisition and tracking in an OFDM receiver that overcomes the problems that are inherent when the symbol time is close to the frame boundary. Yet another need exists for a method and apparatus for performing timing acquisition and tracking in an OFDM receiver that declares when timing has been acquired or when timing has been lost.